1. Field of the Invention
The present invention relates to the arrangement of an electrical heating element as a cooking zone, the cooking zone having as a carrier a ceramic of very high thermal conductivity, in a cutout of a cooking surface consisting of glass-ceramic, glass, ceramic, metal or plastic.
2. Background Information
Cooking appliances having glass-ceramic cooking surfaces are known. In these appliances, the cooking zones are heated, as a rule, by means of electrically operated or gas-operated heating devices arranged below the glass-ceramic cooking surface in the region of the cooking zones. These heating devices may, for example, be electrically operated contact-heating or radiant-heating elements or else gas-jet burners.
One disadvantage is held to be the delayed emission of heat through the plate onto the product to be heated, along with a correspondingly lower energy utilization (efficiency) of the heating medium and, as a result, a longer cooking time.
The use of electrical heating elements as cooking zones or cooking plates, having as a carrier an electrically insulating ceramic which, however, is of very high thermal conductivity, in electrical cooking appliances is known per se, for example from European Patent No. 0,069,298 B1. This publication refers, inter alia, to the fact that silicon nitride is particularly suitable as cooking-plate material on account of its high thermal conductivity, low thermal expansion and, at the same time, high resistance to cyclic temperature stress. According to this European patent specification, the material has high mechanical strength and can therefore be designed as a thin plate. This results in the plate having low thermal capacity, so as also to substantially ensure that the heat supply can be regulated quickly and without any inertia or delay.
The subject of WO 96/09738 is likewise an electrical heating element which has an electrically insulating, thermally conductive carrier consisting of ceramic, in particular of silicon nitride, and has an electrically conductive layer or foil applied to the carrier and provided with electrical contacts, the carrier being designed so as to be plate-like and so solid that it acts as a heat sink.
Due to the high thermal conductivity of the ceramic, the heat flow through the cooking plates onto the product to be heated is particularly high; the heating-up speed, reaction speed and energy utilization are therefore particularly advantageous here.
On the other hand, on account of the very high thermal conductivity of the ceramic, a one-piece cooking surface cannot be used, as is possible in the case of cooking appliances having glass-ceramic cooking surfaces, since the heat would then flow away out of the hot region. In such a case, energy utilization would be impaired and the temperatures permissible on the frame of the appliance would be exceeded. It is therefore necessary to insert such a heating element as a cooking zone, having as a carrier a ceramic of very high thermal conductivity, into a baseplate so as to be thermally insulated.
Furthermore, the high thermal conductivity of the ceramic material prevents multi-circuit cooking zones from being designed with diameters or broiling zones adapted to the cooking utensil and having independently switchable and controllable zones, such as have already been known for years in the case of glass-ceramic cooking surfaces and, in general, are used to good effect. Specifically, here, adjacent zones would also be mutually heated.
Thus, despite the abovementioned advantages, as compared with cooking appliances having glass-ceramic cooking surfaces or zones, cooking appliances, the cooking zones of which are formed solely by ceramic cooking plates of very high thermal conduction, have a whole series of disadvantages.
These disadvantages are taken into account in German utility model 297 02 418.3, the subject of which is a cooking appliance having a glass-ceramic cooking surface with a plurality of cooking zones, at least one of which is designed as a high-speed cooking zone, the cooking zones being heatable essentially by means of electrically operated heating devices, and the high speed cooking zone being formed by a ceramic cooking plate integrated into the glass-ceramic cooking surface, the ceramic cooking plate being capable of consisting of Si.sub.3 N.sub.4 or SiC. According to this utility model, the ceramic cooking plate either is inserted directly into the glass-ceramic cooking surface or is glued into a corresponding cutout in the glass-ceramic cooking surface by means of a silicone adhesive, or the ceramic cooking plate is inserted into a plate consisting of thermally insulating ceramic, of metal or of prestressed glass and the latter plate is, in turn, inserted into a cutout of the glass-ceramic cooking surface.
However, all these assembling and gluing methods have considerable disadvantages in practice and in long-term use. Ceramic plates can have appreciable thermal expansion. Since a cooking plate consisting of ceramic therefore expands during operation, high operating temperatures should not occur when such a cooking plate is assembled together with, for example, brittle materials, such as glass-ceramic, glass or ceramic.
Alternatively, gluing may be carried out with a permanently elastic material. However, these permanently elastic materials are typically also resistant only up to about 300.degree. C. Furthermore, the tempering resistance of ceramic plates is typically around 300 K.
The operating temperature of such ceramic cooking plates is therefore restricted to about 250.degree. C. up to a maximum of about 300.degree. C. However, so that the ceramic cooking plate can be used at these lower temperatures, it is necessary to employ costly special pots having a highly planar bottom which is likewise of very high thermal conductivity.
By contrast, due to the lack of planeness in the bottoms of commercially available utensils, cooking zone temperatures of up to 600.degree. C. are typically required, in order to bring the product to be heated to boiling smoothly. For this purpose, therefore, special temperature sensors and regulating devices can additionally be necessary in the case of cooking appliances having ceramic cooking plates.